Without knowing which head and which cabinet you're talking about and something about the bass that you used when you liked the sound of the bass/amp/cabinet combination, it's really hard to tell you much.

Being pro level gear it can probably sound really good or really bad depending on how you set up the EQ and the way you play.

I love P Basses and don't think I've ever played through an amp that made one sound bad...

Colin Greenwood of Radiohead plays (or played) a p-bass through a GK-800RB head and Ampeg SVT 810 cabs. I think he has a great sound. I'm not sure how much of that is influenced by the Ampeg cabs though.

I played through a GK 400RB head with my not-so-awesome-but-has-much-sentimental-value P bass clone last year. I thought the head worked well for the pretty straightforward sound I wanted.

I have the GK 1001RB-II and I run it through a variety of cabs depending on the venue (mostly Peavey at the moment (1x15BW, 4x10TVX, but have used Ampeg SVT410, PR410, on occasion and I also have an Acoustic 306 (2x15). With the exception of the Acoustic cab which is coming apart at the seams on me, all the other cabs sounded very good with my Fender Precision. But the RBH series or Neo series cabs from GK or a Schroeder are definitely on my list. I have played the RBH 1x15 with the Fender and I thought the tone rocked. Just my .02